As can be seen in FIG. 1, it is known from WO03/087643, which is indicative of the present state of the art, to provide a rotary valve 10 which comprises a generally spherical ball valve element 12 located within a valve housing 16. The ball valve element has a bore 18 which is partially occluded by the provision of one or more impedance assemblies 20 through which are provided tortuous flow passages (not shown). The impedance assembly acts as a pressure control device allowing a controlled pressure drop and thus energy dissipation within the fluid flow. This is beneficial in reducing cavitation, erosion, vibration and noise within the valve assembly.
The problem associated with the presently known impedances applied to bores of rotary valve assemblies is that they are complex, requiring many stacked plates with intricate positioning of openings and apertures. Due to these intricacies, machining accuracies and tolerances are critical, thus increasing production costs and lowering production rates.
Furthermore, known impedance assemblies, when the ball valve element rotates, only gradually present the full inlet of a flow passage to the fluid flow. This consequently results, initially at least, in low fluid flow within a relatively large flow passage. Consequently, the majority of energy dissipation occurs solely at the inlet to the flow passage with very little further energy dissipation occurring through the turns in the passage.
Other known impedance assemblies have multiple flow passages, the inlets of which are fully presented to the fluid flow as the valve element is incrementally rotated. However, this again requires intricate arrangements and high accuracy of machining due to the increased number of flow passages.
The present invention seeks to overcome these problems.